Some materials (for example nylon, certain wool fibers, and elastomers) experience a phenomenon called "stress softening." As a result of this phenomenon, the stress-strain curve of a material prior to being loaded is different from that of the material after being loaded and unloaded. For some materials, the properties of the first loading and any subsequent loading can be dramatically different. Elastomers are subject to stress softening in a behavior known as the "Mullins' Effect."
Many devices include materials subject to stress softening. For example, prosthetic heart valves sometimes include materials subject to stress softening, including elastomers.
Ever since 1950, when blood oxygenators made open heart surgery feasible, it has been possible to treat some forms of heart disease by replacing one of the patient's heart valves with a prosthetic valve. Early heart valve prostheses included ball-and-cage valves and disc-and-cage valves in which a ball or a disc was housed in a cage. One side of the cage provided an orifice through which blood flowed either into or out of the heart, depending on the valve being replaced. When blood flowed in a forward direction, the energy of the blood flow forced the ball or disc to the back of the cage allowing blood to flow through the valve. When blood attempted to flow in a reverse direction, or "regurgitate", the energy of the blood flow forced the ball or disc into the orifice in the valve and blocked the flow of blood.
A bi-leaflet valve comprised an annular valve body in which two opposed leaflet occluders were pivotally mounted. The occluders were substantially rigid and moved between a closed position, in which the two leaflets were mated and blocked blood flow in the reverse direction, and an open position, in which the occluders were pivoted away from each other and did not block blood flow in the forward direction. The energy of blood flow caused the occluders to move between their open and closed positions.
A tri-leaflet valve comprised an annular valve body in which three leaflets were mounted to a portion of the valve body, called a "stent," located at the circumference of the annulus. When blood flowed in the forward direction, the energy of the blood flow deflected the three leaflets away from the center of the annulus and allowed blood to flow through. When blood flowed in the reverse direction, the three leaflets engaged each other in a coaptive region, occluded the valve body annulus and prevented the flow of blood. The valve leaflets were made from tissue, such as specially treated porcine or bovine pericardial tissue, or from man-made materials such as ceramic materials, elastomers or other biocompatible polymers.